1. Technical Field
This invention generally relates to eye protection glasses and more particularly, it relates to the lenses used in sunglasses.
2. Background Art
The spectrum of electromagnetic radiation from the sun which reaches the earth's surface ranges from 0.01 nanometers to over 1 million kilometers in wave length and includes infrared, visible light, ultra-violet light, x-rays, and gamma rays. The visible light range is generally defined as those wavelengths in the 400 to 700 nanometer range.
Visible light from the sun is scattered and reflected as it enters the atmosphere, oftentimes causing a dazzling brightness which can cause visual discomfort. When light strikes a flat surface, part of the light is reflected from this surface and part of the light is absorbed. The light that is reflected can produce a very harsh, bright, dazzling light which is commonly referred to as glare.
Electromagnetic radiation with wavelengths of approximately 800 to 1000 nanometers is part of the invisible infrared spectrum. This is the thermal wavelength which is responsible for the heat from the sun. This heat is also a discomfort to the eyes.
Wavelengths shorter than 400 nanometers fall below the visible light spectrum and represent the ultraviolet light spectrum. Excessive exposure of the eyes to ultraviolet can cause damage to the cornea and the lens of the eye. Most concern over possible damage to the eye involves the band of ultraviolet light known as UVB which are wavelengths from 290 to 315 nanometers. The longer wavelengths of ultraviolet light, from 315 nanometers to 380 nanometers, are called UVA. These wavelengths can also be detrimental, but much less so than UVB.
Part of the UVB, including all wavelengths shorter than 295 nanometers, are absorbed by the cornea, the transparent tissue that covers the colored iris and pupil of the eye. Although ultraviolet rays damage corneal cells, the cornea repairs itself so rapidly that it usually keeps up with the assault. However, prolonged ultra-violet exposure can overwhelm the cornea's repair processes causing temporary blindness. Even so the cornea will usually recover within a few days.
The chief concern about ultraviolet damage, centers on the lens of the eye. Lifetime exposure to ultraviolet contributes to some types of cataracts. The human eye is for all practical purposes, an ultraviolet filter. The lens of the eye absorbs all the UVA plus any UVB not blocked by the cornea. The lens thus shields the light sensitive retina at the back of the eye from the ultraviolet rays, but possibly places itself at risk in the process.
Efforts to reduce the discomfort of bright sunlight or the effects of glare such as wearing a hat or a visor with a wide brim to shade the eyes from the sun have been used for many years. What used to be known as shaded or smoked glasses have also been around for a long time. George Washington had a pair. These smoked glasses or sunglasses as they are more commonly called even pre-date prescription glasses although their popularity did not become evident until the early twentieth century. While these methods served to cut down on the amount of light reaching the eyes, they did little to reduce the glare from reflected surfaces and virtually nothing to reduce the ultraviolet light which reached the eyes.
Thus, sunglasses which were designed to give comfort and protection to the eyes were developed.
All sunglasses have lenses which are manufactured from glass or synthetic materials. These lenses may have the light absorbing dyes and ultraviolet inhibiting chemicals mixed into the lens materials or coated on the surface of the lens. Glass lenses are scratch resistant, non-warping and usually distortion free. Synthetic lenses are light, virtually shatterproof and generally less expensive.
The first polarizing sunglasses were developed by Edwin Land in 1936. When random non-polarized light from the sun reflects off of a flat surface the light waves become directional, or polarized, rather than random. Ordinary non-polarized light is a grouping of electromagnetic waves that can ripple in a variety of directions, polarized light waves are restricted to one plane. This causes a hot spot of intense glare. Glare is polarized light.
Polarized sunglasses filter out this glare by the use of a polarized light filtering film commonly made of polyvinyl alcohol dyed with iodine. This film is constructed so that its crystal molecules are aligned in parallel lines. When the polarized light impacts the polarizing film, only light waves which lie in the same vertical plane as the crystals in the polarizing film are allowed to pass through the membrane. The light which passes through this film is then made to pass through a second polarizing film that is at right angles to the first polarizing film. Thus, every possible path for the light is blocked and glare is eliminated.
One method to reduce bright light has been to add light absorbing dye to the lenses material as in the case of synthetic lenses which readily accept tinting mediums or to coat the surfaces of the lenses as is done with glass lenses which do not accept tinting mediums very easily. Differing amounts of this light absorbing dye is added to the lens material or coated on the surface producing a lighter or darker lens which effectively blocks out some of the visible light thus reducing or eliminating bright light. Some manufacturers utilize a tinting process which produces gradient or multi-gradient lenses. These lenses are darker at the top than at the bottom or they may be darker at the top and the bottom and lighter in the middle. In this manner the top filters are high to absorb bright light from the sun while the bottom filters are used to absorb direct glare and bright light from water or road surfaces. WOODARD, ET AL., U.S. Pat. No. 4,943,140, describes a windshield for automobiles which is made of laminated glass comprising thermal plastic inner layers sandwiched between two layers of glass. This inner layer has an integral marginal band of light absorbing dye above the driver's direct line of vision for the windshield to reduce sunlight glare. This band is graduated in intensity, being greatest near the upper peripheral of the inner layer when in place in a windshield and greatly diminishing into an almost imperceptible level at the lower edge of the band. However, as described this particular invention is for a windshield for an automobile.
A common feature on many sunglasses is a mirrored surface. Mirroring is accomplished by applying a thin coating of a transparent light reflective metal oxide coating to the lens which reflects some of the bright light and also reflects much of the heat created by the infrared rays. This feature also reflects some of the ultraviolet light.
HUFFER, U.S. Pat. No. 4,902,081, teaches a window for a building which uses two panes of glass in a juxtaposed relationship providing an air space in between. On the inside surface of the outer pane of glass, there are a plurality of metal coatings applied to absorb visible light and infrared radiation. However, this invention differs from the present invention in that it contains a space between the two glass plates. Also, the metal coatings on the inner surface of the outer glass pane serve to absorb rather than to reflect the visible light and infrared radiation.
A more recent development in sunglass technology is photochromic lenses, ARMISTEAD, et al. U.S. Pat. No. 3,208,860. Photochromic lenses have the ability to adjust their density in relation to the sun. These glass lenses have submicroscopic silver halide crystallites impregnated in the lens material. This silver halide ingredient darkens under the action of ultraviolet radiation to reduce the optical transmittance of the glass. When the source of ultraviolet radiation is removed, the silver halide crystallites return to their original color state, restoring the optical transmittance to its original level. In glass lenses, this sequence of darkening and fading can be repeated indefinitely without fatigue or loss of photochromic properties. However, attempts to introduce photochromic chemicals into synthetic lenses has been largely unsuccessful because the photochromic properties disappear in a relatively short time.
While these lenses may all be very useful and provide adequate eye protection for their individual and specific uses, what is needed is a lens for sunglasses that is strong and shatter resistant which cuts down on visible light, reduces glare, reflects heat and protects the eyes from harmful ultraviolet light waves.
It is therefor an object of this invention to provide a lens for sunglasses that satisfies these needs.